Anisotropic magnetoresistive (AMR), or simply magnetoresistive (MR), sensors are used in a variety of automotive applications. They are used, for example, as angular sensors in throttle valves, rotational speed sensors in Automatic Breaking Systems (ABS), automated and automatic transmission systems, and as motion sensors in security systems.
Recently there has been a demand to miniaturise the sensors and to reduce their cost, while improving performance to compete with giant magnetoresistive (GMR) technology. The largest element of a conventional AMR rotational speed sensor is the biasing magnet which is glued on the outside of the package. This biasing magnet is also rather costly. It is used to provide a bias magnetic field to the sensor, stabilise magnetic domains, and prevent magnetisation flipping. AMR sensors are often also provided with metal lines running over the elements, known as barber poles, which serve to linearise the sensor signal.
There have been several proposals for performing these functions without requiring a biasing magnet. These proposals include the superheterodyne and differential superheterodyne principles in which an alternating current (a.c.) excitation field created by an integrated coil in a direction perpendicular to the sensor strips (the Y direction) is mixed with the a.c. external field (to be measured) using the non-linear characteristic of the mganetoresistive sensor elements. The coil is a layer of a straight conductor that lies directly above the magnetoresistive elements, separated by a thin non-magnetic insulator layer. It creates a magnetic field in the plane of the magnetoresistive sensor elements, in a direction perpendicular to their longitudinal axis.
FIG. 1 shows such a sensor comprising a lower layer with three parallel rows of three magnetoresistive segments 1 electrically connected in series by connections 2, and an upper layer (separated from the lower layer) in which a coil 3 is formed. The magnetic field generated by the coil is shown by the arrows marked “H” and can be seen clearly in the cross-section. To generate the magnetic field, an a.c. excitation current is passed through the coil 3. One end of the magnetoresistive element is coupled to ground and the other end is coupled to an output terminal.
In the superheterodyne principle, an a.c. excitation current, having a frequency about 10 times higher than that of the external field to be detected, is caused to flow through the coil to create an a.c. excitation field on the magnetoresistive sensor elements. This field is mixed with the external field inside the magnetoresistive elements (by virtue of their quadratic transfer function-resistance change is a quadratic function of the applied magnetic field in the Y direction).
After mixing, the resulting signal has a spectrum with components d.c. and at frequencies of ωexc−ωext, ωexc+ωext and 2ωexc (where ωext and ωexc denote the frequencies of the external field to be detected and of the excitation field created by the coil respectively). After filtering out the frequency component at 2ωexc, the signal contains the spectrum components at ωexc−ωext and ωexc+ωext, which convey the useful information. The signal is then mixed again with a signal at a suitable frequency to bring one of these two components into a base-band frequency range, and the signal corresponding to the external field can then be extracted easily.
In the differential superheterodyne principle, two sensor arrangements identical to those of the superheterodyne principle are placed close to each other and the excitation currents flowing in the two coils are opposite in direction in order to create a 180 degree phase difference in the excitation magnetic field. After mixing in the magnetoresistive elements, signals from the two sensor arrangements are subtracted to result in a signal that contains only the two side-band components ωexc−ωext and ωexc+ωext and the direct current (d.c.) component. The useful information can be extracted in the same way as in the superheterodyne principle (i.e. by mixing), without having to use band-pass filters.
For both superheterodyne and differential superheterodyne methods, the useful information is conveyed by the two components ωexc−ωext and ωexc+ωext. It is very important that there should be no interfering frequency components getting into or close to this bandwidth.
These methods have a number of advantages. For example, they are highly sensitive, no bias magnet or barber poles are needed, the output signal has no offset, they are independent of temperature, and they have a wide dynamic range of input field magnitude.
However, there is a problem with both the superheterodyne and differential superheterodyne methods. Specifically, there is a strong frequency component at ωexc, which lies in the middle of the two useful components ωexc−ωext and ωexc+ωext and does not contain useful information. This interfering component appears due to unwanted capacitive and/or inductive coupling between the coil and the magnetoresistive elements.
An illustration of the mechanism leading to capacitive coupling is shown in FIG. 2. A source 4 sends an a.c. excitation current through the coil 3, and a source 5 sends a d.c. bias current through the magnetoresistive element. The a.c. signal in the coil 3 is coupled via the capacitance between the coil 3 and the magnetoresistive element (shown as three discrete capacitors rather than a distributed capacitance in FIG. 2 for ease of illustration), giving rise to an a.c. leakage current in the magnetoresistive element. This current finds its way to ground by passing through the resistance of the magnetoresistive segments 1, giving an unwanted a.c. voltage contribution to the output voltage at the output terminal, VA.
An illustration of the mechanism leading to inductive coupling is shown in FIG. 3. The a.c. signal in the coil gives rise to an a.c. voltage contribution throughout the magnetoresistive element due to the mutual inductance between the coil 3 and magnetoresistive element. This unwanted a.c. voltage adds to, and interferes with, the wanted signal at the magnetoresistive element output terminal.
Another problem is electrical mismatch between the coils in the differential superheterodyne principle. This principle only works when the phases of the excitation magnetic field created by the two coils are exactly in anti-phase. However, in practice, it is difficult to fulfil this requirement, due to impedance mismatch between the coils.
It has been proposed to drive the magnetoresistive sensor with an a.c. current that has the same frequency and phase as the excitation current in the coil. In principle, this could cancel the capacitive coupling because at corresponding points along the coil and the magnetoresistive element there is no potential difference. However, in practice it is extremely difficult to provide good phase matching because the impedance of the coil and the magnetoresistive element are not the same.